HYDROCARBON CHARACTERIZATION AND WEATHERING OF OILED INTERTIDAL SEDIMENTS ALONG THE SAUDI-ARABIAN COAST 2 YEARS AFTER THE GULF-WAR OIL-SPILL

Two years after the 1991 Gulf War oil spill, shoreline areas of Dawhat al Musallamiyah and Dawhat ad Dafi along the Saudi Arabian Coast impa cted by the largest oil spill in history were revisited to assess the changes, from weathering, in hydrocarbon composition of residual oil i n intertidal sediments. This was done to determine the effect of diffe rent geomorphic habitats from exposed to sheltered on the extent of we athering of these oiled sediments. This 1993 study was a continuation of the nearshore geochemistry processes study conducted as part of the scientific expedition on the RN Mt. Mitchell in the Arabian Sea in 19 92. Over 70 surface and subsurface shoreline sediment samples were ana lyzed for saturated hydrocarbons (SHCs), defined as n-alkanes from C-1 0 to C-32 and selected isoprenoids, and polycyclic aromatic hydrocarbo ns (PAHs), defined as individual and alkyl homologues of the two-throu gh six-ringed PAHs including the sulfur heterocyclic compounds. The ex tent of weathering of hydrocarbons over the two-year period was determ ined for sediments in each of the different shoreline habitats based o n SHC and PAH distribution pattern, and diagnostic ratios, such as rat ios of the C-2-and C-3-alkyl homologues of the dibenzothiophenes and c hrysenes/benz(a)anthracenes. A classification depicting four stages in weathering (from 1 [fresh] to 4 [advanced]) was established for both SHCs and PAHs. Some sediments contained relatively fresh oil that show ed no change in SHC or PAH distributions over the two years in the env ironment. Other sediments showed extensive weathering, involving multi ple weathering stage changes. For SHCs, approximately 66% of the oiled sediment samples had progressed to the most advanced weathering stage (Stage IV) by the end of the second year. In contrast, only 25% of th e oiled samples had PAHs at full;Stage IV weathering. For the SHC hydr ocarbons, changes in SHC distributions looked to be principally caused by selective microbial degradation of the n-alkanes. Weathering:of sp illed oil PAHs was a combination of losses of smaller PAH families (na phthalenes) and smaller carbon alkyl groups within a PAH family. The l east weathered oiled sediments were predominantly from subsurface sedi ments in habitats with surface seals or in sheltered muddy areas which had reduced exposure from physical weathering processes involving air , sunlight, tidal flushing, and reworking by waves. Seventy percent of the subsurface samples contained residual oil that was relatively fre sh (Stage I or II), whereas 20% of the surface samples had relatively fresh residual oil. The heavily oiled subsurface sediments, even in ex posed habitat environments, showed the least weathered oil, particular ly where liquid oil remained trapped under hardened near-surface oil r esidual (crust and pavement). Extremely weathered oiled sediments were found mostly in exposed or moderately exposed near-surface sediment h abitats. PAHs in oiled sediments were examined for selective degradati on of certain alkyl groups or isomers within an alkyl group by microbi al action. PAH distributions of the residual oil indicated that select ive microbial degradation of alkyl groups was negligible and the loss of PAHs by any weathering mechanism (microbial, physical, chemical) lo oked to follow, at least in this two-year time period after the spill, the weathering pathway predicted by the physical processes (evaporati on). (C) 1998 Elsevier Science Ltd.